Polyimide is a kind of polymer containing imide group in the main chain; the phthalimide structure embedded in the chain has drawn the most attention. As a kind of special engineering material, the polyimide has been widely applied in aviation, aerospace, microelectronics, nanomaterial, liquid crystal display, separation membrane, laser, etc. Recently polyimide has been listed as one of the most promising engineering plastics in the 21st century by many countries. No matter as structural material or as functional material, the great application prospect of polyimide has been well understood, it was called “protion solver”, and it has been considered that “there will be no today's microelectronics without the polyimide”
Therein the polyimide foams have advantages of good fireproof performance, low smoke density index and good thermal insulating property. Thanks to these properties polyimide foams have been widely applied in aviation, aerospace, marine, and transportation, etc. And they also can be used in civil building for sound and heat isolation purpose. However the application is limited because of high preparation cost and complex process.
There are two main methods for preparing the polyimide foams: one is that polyimide foams are synthesized from aromatic dianhydride and aromatic diamine by two-step synthesis; the other method is that aromatic dianhydride directly reacts with the isocyanate to produce polyimide foams.
The first method consists of two stages: in the first stage dianhydride reacts with diamine by way of condensation polymerization in polar solvent, thereby forming solution. At low temperature the volatile solvent is heated to evaporate and the solution's viscosity increase, to even form solid. The remaining solvent from the products above is working as foaming agent. The second stage is to remove the water which is produced in reaction and the alcohol at high temperature; the imidization reaction happens during the heat treatment stage, associating by solvent evaporation. The shortage of the two-step method is very obvious: the reaction goes very slowly; the equipment and the process are very complex; so the preparation cost is very high; the solvent used is not friendly to the environment.
In the second method, an aromatic dianhydride is firstly dissolved in a high boiling point solvent. The solution then quickly reacts with the isocyanate to form the polyimide foams. In this method, a high boiling point solvent is necessary; the density of the products cannot be well controlled; also it is impossible to produce the foam in a continuous way.
The method as discussed in CN101812232A utilized the aromatic dianhydride or aromatic tetracid, isocyanate and foaming agent, etc. to immediately react in a polar solvent, thereby cure into polyimide foam. The patent utilized high boiling polar solvent such as N,N-dimethylacetamide. However the process is only suitable for intermittent production.
Patent CN102127225A disclosed another method for producing the rigid polyimide foams. Aromatic dianhydride or aromatic ester reacted with polyhydric alcohol in polar solvent, thereby forming polyimide foams precursor which would react with isocyanates in a mould forming polyimide foam intermediately. Rigid polyimide foams are obtained after the intermediate is cured by doing heat treatment in microwave. In this patent, since the solvent is not involved in the reaction process, it needed to be removed. The process is relatively complex.
Patent CN102942668A disclosed a method for producing the polyimide composite material by filling the functional filler. The method employed aromatic dianhydride or aromatic diamine. After aromatic dianhydride is esterified, promoters, such as catalyst, foaming agent and functional filler, are added, thereby forming the precursor solution. Then the precursor solution reacted with isocyanate to produce polyimide foams. In this method the solvent doesn't participate in reaction process and needed to be removed. Microwave or heat treatment process is employed during solidification. The process is relatively complex and cannot satisfy the requirement of continuous production.